I’ll give you a thumbnail sketch (no pun intended): let’s say you want just a single button to play different video in it than what it’s linked to.
1. Save your menu as a file (Menu > Save As File), you’ll import this PSD file into PPro later.
2. Create a new PPro project, either standard 4:3 or 16:9, depending on your DVD project needs.
3. Import the PSD file you created in step 1
4. Import the video that you want to play in the thumbnail
5. Add it to the next video track, then scale it down so it fits in the button
6. Make sure both clips are of equal length, then export an AVI from PPro with Optimize Stills unchecked (important)
7. Import this AVI into Encore
8. Alt-drag it into your menu
9. File > Render Motion Menus
Hope it helps,
Joe